I would like to be able to call the Excel REST API using app only permissions.

My use case is to have excel workbooks with models that have complex calculations. I could then use then call the Excel REST API from say an Azure Function using a Managed Identity that first populates some cells (that are parameters so to speak) and then calculates the workbook and gets the result from another cell, so basically I can delegate the "programming" of the complex model to a finance user who knows how to build complex models in excel.

I would effectively be using the Excel workbook as my custom calculation engine, allowing me to build some non-trivial code as a (straight-forward to any finance expert) model, and I can simply use the Excel REST API to "return" the result of the calculation.